U.S. Patent Publication No. 2012/0034150 A1, published Feb. 9, 2012, the disclosure of which is hereby incorporated herein in its entirety by this reference, discloses background information hereto.
Additional information is disclosed in the following documents, the disclosure of each of which is hereby incorporated herein in its entirety by this reference:                1. International Application No. PCT/US2013/00072, filed Mar. 15, 2013, for “Methods and Structures for Reducing Carbon Oxides with Non-Ferrous Catalysts,” which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/624,702, filed Apr. 16, 2012, in the name of Dallas B. Noyes;        2. International Application No. PCT/US2013/00076, filed Mar. 15, 2013, for “Methods and Systems for Thermal Energy Recovery from Production of Solid Carbon Materials by Reducing Carbon Oxides,” which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/624,573, filed Apr. 16, 2012, in the name of Dallas B. Noyes;        3. International Application No. PCT/US2013/00077, Mar. 15, 2013, for “Methods for Producing Solid Carbon by Reducing Carbon Dioxide,” which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/624,723, filed Apr. 16, 2012, in the name of Dallas B. Noyes;        4. International Application No. PCT/US2013/00073, filed Mar. 15, 2013, for “Methods and Reactors for Producing Solid Carbon Nanotubes, Solid Carbon Clusters, and Forests,” which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/624,753, filed Apr. 16, 2012, in the name of Dallas B. Noyes;        5. International Application No. PCT/US2013/00071, filed Mar. 15, 2013, for “Methods for Using Metal Catalysts in Carbon Oxide Catalytic Converters,” which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/624,848, filed Apr. 16, 2012, in the name of Dallas B. Noyes;        6. International Application No. PCT/US2013/00081, filed Mar. 15, 2013, for “Methods and Systems for Capturing and Sequestering Carbon and for Reducing the Mass of Carbon Oxides in a Waste Gas Stream,” which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/624,462, filed Apr. 16, 2012, in the name of Dallas B. Noyes;        7. International Application No. PCT/US2013/00078, filed Mar. 15, 2013, for “Methods and Systems for Forming Ammonia and Solid Carbon Products,” which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/671,464, filed Jul. 13, 2012, in the name of Dallas B. Noyes; and        8. International Application No. PCT/US2013/00079, filed Mar. 15, 2013, for “Carbon Nanotubes Having a Bimodal Size Distribution,” which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/637,229, filed Apr. 23, 2012, in the name of Dallas B. Noyes.        
Solid carbon has numerous commercial applications. These applications include the longstanding use of carbon black and carbon fibers as filler material in tires, inks, etc.; use of various forms of graphite (e.g., pyrolytic graphite in heat shields); and innovative and emerging applications for carbon nanotubes (CNTs) and buckminsterfullerenes. CNTs are valuable because of their unique material properties, including strength, current-carrying capacity, and thermal and electrical conductivity. Current bulk use of CNTs includes use as additives to resins in the manufacture of composites. Research and development on CNTs is continuing, with a wide variety of applications in use or under consideration. One obstacle to widespread use of CNTs, however, has been the cost of manufacture. Conventional methods for the manufacture of solid carbon typically involve the pyrolysis of hydrocarbons in the presence of a suitable catalyst. Hydrocarbons are typically used as the carbon source, due to abundant availability and relatively low cost.
Carbon oxides, particularly carbon dioxide, are abundant gases present in ambient air and point-source emissions, such as exhaust gases generated by hydrocarbon combustion or off-gases generated by various manufacturing processes. Conventional aluminum manufacture, for example, involves the reduction of alumina (Al2O3). The process typically uses sacrificial carbon anodes to both deliver the electrical energy, and the carbon that reduces the aluminum oxides in the ore to produce the aluminum and carbon dioxide. Approximately two tons of carbon dioxide are produced for each ton of alumina that is reduced. Similarly, conventional steel manufacture involves the reduction of the oxides of iron present in iron ore or scrap iron. Carbon (in the form of coke, or in the form of sacrificial carbon anodes) is typically used as the reducing agent in the manufacture of steel, producing large amounts of carbon dioxide. Cement manufacture involves calcination, heating a raw material such as limestone (calcium carbonate) in a kiln, which liberates carbon dioxide. In addition to the carbon dioxide formed during calcination, carbon dioxide may be formed by the combustion of fuels (e.g., coal, natural gas, etc.) used to drive the calcination process, which may be either direct (combustion occurring within the calciner) or indirect (combustion occurring outside the calciner, with resultant heat transferred to the calciner). Worldwide, cement plants contribute about 5% of the total carbon dioxide emitted to the atmosphere from industrial processes. Cement manufacture is also associated with emission of various other waste products, including NOx (primarily NO), sulfur compounds (primarily SO2, with some sulfuric acid and hydrogen sulfide), hydrochloric acid, and particulate matter, including dust. The concentration of carbon dioxide in flue gases from cement plants is typically 15%-30% by volume, significantly higher than in flue gases from power plants (3%-15% by volume).
Concerns about greenhouse gases are encouraging industry and governments to find ways to minimize carbon dioxide production and its release into the atmosphere. Some methods for reducing carbon dioxide emissions involve capture and sequestration of the carbon dioxide (e.g., by injection into a geological formation). These methods, for example, form the basis for some “green” coal-fired power stations. In current practice, however, capture and sequestration of the carbon dioxide entails significant cost.
There is a spectrum of reactions involving carbon, oxygen, and hydrogen wherein various equilibria have been identified. Hydrocarbon pyrolysis involves equilibria between hydrogen and carbon that favors solid carbon production, typically with little or no oxygen present. The Boudouard reaction, also called the “carbon monoxide disproportionation reaction,” is the range of equilibria between carbon and oxygen that favors solid carbon production, typically with little or no hydrogen present. The Bosch reaction is within a region of equilibria where all of carbon, oxygen, and hydrogen are present under reaction conditions that also favor solid carbon production.
The relationship between the hydrocarbon pyrolysis, Boudouard, and Bosch reactions may be understood in terms of a C—H—O equilibrium diagram, as shown in FIG. 1. The C—H—O equilibrium diagram of FIG. 1 shows various known routes to solid carbon, including, carbon nanotubes (“CNTs”). The hydrocarbon pyrolysis reactions occur on the equilibrium line that connects H and C and in the region near the left edge of the triangle to the upper left of the dashed lines. Two dashed lines are shown because the transition between the pyrolysis zone and the Bosch reaction zone may change with reactor temperature. The Boudouard, or carbon monoxide disproportionation reactions, occur near the equilibrium line that connects O and C (i.e., the right edge of the triangle). In this zone, the Boudouard reaction is thermodynamically preferred over the Bosch reaction. The equilibrium lines for various temperatures that traverse the diagram show the approximate regions in which solid carbon will form. For each temperature, solid carbon may form in the regions above the associated equilibrium line, but will not generally form in the regions below the equilibrium line. In the region between the pyrolysis zone and the Boudouard reaction zone and above a particular reaction temperature curve, the Bosch reaction is thermodynamically preferred over the Boudouard reaction.
The use of carbon oxides as a carbon source in production of solid carbon has largely been unexploited. The immediate availability of ambient air may provide economical sources of carbon dioxide for local manufacture of solid carbon products. Because point-source emissions have much higher concentrations of carbon dioxide than ambient air, however, they are often economical sources from which to harvest carbon dioxide.